|In simple terms, the compression stroke of the Miller-cycle engine is shortened with results in a low compression ration, yet a high expansion ratio.
In order to grasp this and other aspects of the Miller-cycle, one has to go back and have a look at some of the basic principles of internal combustion engine operation. There are four areas worth reviewing.
Engine Size vs Frictional Losses
When the displacement of an engine is reduced, there is a substantial reduction in frictional losses. For example, 25 percent less friction is produced rotating a particular engine that has its displacement reduced by 30 percent. An automatic offshoot of such downsizing is an improvement in fuel efficiency of around 10-15 percent.
Theoretical vs Actual Compression Ratio
The theoretical compression ratio is simply a comparison of the volume above the piston when it is at bottom dead centre (BDC), to the volume above it at top dead centre (TDC). However, in practice, the actual compression ratio is determined by the valve timing, since the real compression stroke does not begin until the intake valve closes. Similarly, the length of the power (expansion) stroke is also determined by the opening point of the exhaust valve.
With the fairly symmetrical valve timing being found in most engines these days, these two strokes are approximately the same. This means that the actual compression stroke is roughly equal to the expansion stroke.
By increasing the compression ratio, the thermal efficiency of an engine is also increased. However, along with this efficiency gain comes higher combustion pressures and temperatures. These characteristics are usually accompanied by two well known "bad guys" Oxide of Nitrogen (NOx) emissions and knock.
NOx is produced as a result of combustion pressures and temperatures greater than 1,300 deg C. At these temperatures the normally inert Nitrogen (78 percent by volume of intake air), reacts with oxygen to form oxides (nitrogen dioxide and nitrogen monoxide).
Knock is caused when part of the air/fuel charge is ignited spontaneously by the effect of heat and pressure and not the spark plug as Otto intended. This produces two flame fronts in the combustion chamber which can result in serious engine damage.
There are two important things to note here. Firstly, knock is affected by the gas temperature at TDC of the compression stroke. Secondly, most of the gain in thermal efficiency from increases in compression comes mainly from the events that occur on the expansion stroke (more push on the piston). Only a little is gained from the actual increase in compression ratio.
This refers to the energy required to rotate an engine during two of the three non-power producing strokes
- pumping air in and pumping exhaust gases out (but does not include frictional losses). It is a term that describes the efficiency of intaking and exhausting the charge. If the piston does less work in taking and exhausting, less power robbing pumping losses are produced.
One of the reason the original Otto-cycle had the exhaust valve opening brought forward (before BDC) is to allow the residual exhaust gas pressure (which, once the piston is half way down the power stroke is too low to provide much push on the piston) to expel itself and not have to rely on the piston to pump it all out, creating further pumping loss. This modified (Otto) valve timing allows around 50 percent of the exhaust gases to be expelled "for free" (no pumping losses incurred in getting rid of half of the exhaust gas). A throttled engine (eg cruising with high manifold vacuum) has high pumping losses since a vacuum is not produced for free; energy is consumed in doing so. Some experimental variable displacement engines reduce the number of working cylinders (switching some off by holding the valves open) under partload to reduce manifold vacuum and therefore pumping losses.
The term volumetric efficiency refers to the ability of an engine to fill its cylinders with a volume of air equal to their displacement (100 percent Ve). The greater the Ve then the greater will be the output of that engine. Engine manufacturers go to great lengths to "tune" their engine design and obtain the greatest Ve. This involves a lot of research into gas flow - including manifold and port design - as well as valve timing and lift, together with multiple valves and combustion chamber design.
The easiest way to make dramatic improvements in Ve is to add an external device such as a supercharger or turbocharger. Its job is to "force feed" as much air as possible into each cylinder. But, as with increased compression ratio, excessively high combustion pressures and temperatures may be produced by forced induction. These can work against our intent to produce a powerful but clean engine.
The most common method to overcome this problem is to use an intercooler (as well as lowering the compression ratio). An intercooler is an air-to-air heat exchanger that has the ability to reduce air intake temperature (after the supercharger) by at least 50 deg C. This helps keep combustion temperatures to a safe level.
The modern internal combustion engine is a finely balanced mixture of all these (and many more) conflicting requirements.
Miller-cycle Technical Details
There are basically four means that the Miller-cycle uses to obtain its increased efficiency.
(from late closing of the intake valve)
- Smaller engine (lower displacement)
- reduced compression stroke and pumping losses
- cooler intake charge (intercooled air)
- combustion improvements
The graph below indicates the fuel efficiency increase as displacement is decreased. The horizontal axis begins at 1.0 which compares to a 3.3L's fuel efficiency, whilst 0.7 indicates a 30 percent reduction in displacement (down to 2.3L). The two curves represent the changes in efficiency gain with load changes (the greatest being at 20 percent load).